March 2007
doc.: IEEE 802.22-07/0063r2
IEEE P802.22
Wireless RANs
Status of PHY parameters discussion
Date: 2007- 03-07
Author(s):
Name
Gerald Chouinard
Company
Communications
Research Centre,
Canada
Address
3701 Carling Ave.
Ottawa, Ontario
Canada K2H 8S2
Phone
email
613-998-2500
gerald.chouinard@crc.ca
Abstract
This contribution tries to summarize the status of the discussion on the 802.22 PHY parameters as
of the end of the January 2007 session in London, U.K. for the purpose of focussing the
discussion that will take place over teleconference calls between the two sessions so that
finaldecisions can be made before or at the Orlando March session so that an agreed set of PHY
parameters can be include in the Draft 1.0 for Working Group Balloting.
The state of discussion on each key PHY parameter is given along with some observations from
the perspective of the author and a “way forward” is proposed to try to accelerate the consensus
process.
Notice: This document has been prepared to assist IEEE 802.22. It is offered as a basis for discussion and is not binding on the
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patent application) might be incorporated into a draft standard being developed within the IEEE 802.22 Working Group. If you
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Submission
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Gerald Chouinard, CRC
March 2007
doc.: IEEE 802.22-07/0063r2
Status of the PHY parameters discussion and proposed way forward
This document tries to summarize the key items that are left to be discussed before a final set of
parameters for the PHY layer of the 802.22 WRAN standard can be adopted. For each item, a summary
of the discussions so far and some observations from the author are given and a way forward is
proposed. These key items are listed in order of priority for the decisions since the earlier items will
need to be known to take appropriate decisions on the later ones.
1. Synchronization of the CPE
The CPEs have to synchronize quickly with the base station and keep their time and frequency
synchronization during active operation. When a CPE initially associates with a base station, some
delay in synchronization can be acceptable (say less than one second). This allows a CPE to
progressively converge on a proper time and frequency synchronization based on a number of frame
headers (or super-frame headers).
Once the synchronization in time and frequency has been achieved, the CPE needs to maintain this
synchronization within the required tolerances. If the synchronization is lost, re-synchronization needs
to be achieved within a delay acceptable for the types of services to be provided. The most demanding
service will likely be real-time applications such as VoIP which requires a maximum latency of 20 ms.
This means that the re-synchronization will need to be achieved within two frames of 10 ms.
Observations: ETRI’s simulations demonstrated that synchronization can be acquired with one
preamble symbol containing 2 PN-sequences. Monisha Gosh noted that this may need a rather complex
synchronization scheme and synchronization robustness could be improved by using a super-frame
preamble symbol containing 4 PN-sequences to increase the capture range of the synchronization
algorithm. ETRI’s simulations include synchronization based on both fractional as well as integer subcarrier spacings, therefore claimed not to be limited in capture range. The use of a more robust
preamble at the super-frame header would not help to maintain the synchronization for real-time
applications like VoIP since its repetition would only occur every 16 frames.
Way forward: A decision has to be taken as to whether a single symbol frame preamble is sufficient or
two preamble symbols will be needed. As noted above, more robust synchronization at the super-frame
level will not help. The trade-off is between system capacity overhead, synchronization robustness and
synchronization scheme complexity. Efforts should be made to keep only one option to keep the first
draft standard as simple as possible. Note that there is no need for such synchronization requirement on
the upstream since it is assumed that once the CPE is in lock with the base station, its burst received at
the base station will also be synchronous with the base station operation.
Consensus was reached on Feb. 1st that a single preamble symbol per frame will be sufficient. The
discussion on the need for a more robust preamble (one 4-sequence symbol and two 2-sequence
symbols) for the super-frame is still open. More evidence is needed on its need given that it would not
help real-time streaming such as VoIP but may allow more robust synchronization at edge of coverage
and represents 0.7% loss in system throughput. See also comment #491, p. 58.
2. Frame preamble structure
In order to achieve proper time and frequency synchronization, different PN-sequences are proposed for
the frame preamble symbol. A repetition of two PN-sequences allows every other carriers to be
modulated with null carriers in between to acquire a tight frequency synchronization. If a repetition of
Submission
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Gerald Chouinard, CRC
March 2007
doc.: IEEE 802.22-07/0063r2
three PN-sequences is used as in the case of the 802.16e standard, one over three carriers is modulated,
allowing a somewhat less tight synchronization. A repetition of 4 PN-sequences will modulate one
carrier over 4, resulting in somewhat looser frequency synchronization but it is claimed that the capture
range would be better and would allow for simpler synchronization schemes. Another aspect is the fact
that if more carriers are modulated in the preamble, more energy will be available at the CPE to
synchronize.
Observations: The ETRI simulations were done assuming a single preamble symbol with 2 PNsequences. Eli Sofer claims that a repetition of 3 PN-sequences as used in 802.16e shoud be used
whereas Monisha Gosh claims that a preamble symbol with 4 PN-sequences followed by a preamble
symbol with 2 PN-sequences would be needed.
Way forward: After a decision has been taken on the number of frame preamble symbols needed as
indicated in section 1, a decision on the number of PN-sequences has to be taken on evidence of the
trade-offs involved. (Comment: #96, p.58)
Once this is done, then, a decision has to be taken on the actual sequences to be used (actual sequences
or initial vectors and generation polygons, e.g., 802.16e sequences vs. binary m-sequences as proposed
by Monisha Gosh), the number of orthogonal sequences required, the acceptable level of PAPR and
cross-correlation. (Comment: #58, p.57)
Edward Au presented doc. #070002r2 on polyphase CAZAC sequences for low PAPR frame preambles.
Larger is the decimation (2,3,4), higher is the PAPR. Edward Au, Wai Ho, Monisha Gosh and Eli Sofer
to develop a table comparing the PN-sequences from 802.16, M-binary and polyphase to compare
complexity in generating and processing for F & T sync and channel estimation, PAPR and crosscorrelation.
[8 Feb.07] Eli: 3 PN sequences: three repetition of the PN sequence is proposed since it will occupy 1 over 3
carriers in the preamble symbol, thus allowing the transmission of three simultaneous preamble sequences for the
three sectors using the three different subcarrier sets. Three different sets of FHC and US-DS-MAP will be
needed for the three sectors. Eli proposed to concatenate the information for the three sectors in TDM resulting
in the need for 2 FCH and Mapping symbols.
[15 Feb.07] Three step decision process:
- Decide on the number of PN sequence repetitions in the preamble symbol (2 or 3)
- Decide on the type of PN sequences (CAZAC or binary)
- Decide on which specific set of sequences and the number of orthogonal sequences needed.
[22 Feb.07] Eli continued his presentation of the rationalization for the three sectors implementation,
showing that 4 sectors is not good. He also reviewed the ranging process and presented the results of
simulations on the 3x3 tiling structure which are not very good. Averaging over subsequent frames or
selecting the better areas of the channels for allocating the subcarriers (AMC) would be needed to
improve the performance. The 7x1 tiling seems to be more appropriate for WRAN channels.
[28 Feb.07] Wai Ho Mow presented the latest version of the CAZAC polyphase sequence for reduction
of the preamble PAPR. The question is whether the preamble can be boosted or not. An email was sent
to Carl to clarify the averaging period over which the 4 W EIRP limit will be measured. The depth of
interleaving for DTV is about 150 usce. The peaks should therefore be less than 150 usec. Thus the
averaging would likely be at the symbol period. If that is the case, there is no room for boosting the
preamble and the extra PAPR reduction provided by CAZAC sequences would not be needed. The 1/3
sectorized preamble may have an impact on the synchronization reliability. Also, 1 over 3 sub-carriers
Submission
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Gerald Chouinard, CRC
March 2007
doc.: IEEE 802.22-07/0063r2
in the frequency domain needs more than a simple 3 repetition PN-sequence. Ramon to ask Motorola on
their experience with the 3 PN-sequence in implementing 802.16e.
[8 March 07] Eli and ETRI to present results on 3-sequence. Debrief on 2 versus 3 repetition sequence
and vote taken. Debrief from both sides on CAZAC versus Binary sequences and vote to be taken.
3. Channel training
In order for the WRAN system to track the transmission channel variations, channel training information
needs to be included as part of the symbols to be transmitted. Two trends exist in providing such
training information. One is based on what is being done in 802.11 with the transmission of training
symbols at the beginning of a burst whereas the other one is based on what is being done in 802.16 with
the transmission of pilot carriers spread over a number of symbols. The amount of overhead
information that is needed to allow reliable transmission will depend on the characteristics of the
transmission channel, i.e., its time spread and frequency spread characteristics.
The channel training will have different requirements in the downstream direction than in the upstream
direction. The signal received by a CPE from a base station contains all the carriers of the multiplex
whilst the signal received by the base station in the upstream direction only contains a subset of these
carriers that can be spread over the entire 6 MHz channel. Proper interpolation between carriers can be
done to fully characterize the channel in the downstream whereas this is not possible in the upstream
direction.
Note that even if only training symbols were to be used at the beginning of transmission bursts, some
pilot carriers would still be needed within the burst to track the phase of the carriers due to phase noise
in the receiver.
Observations: The state of discussions on this matter is summarized in the spreadsheet: 22-06-026402-0000_OFDMA_Parameters.xls. It was assumed that the channel training in both downstream and
upstream directions would be based on 240 pilot carriers per symbol, allowing all carriers to be visited
over 7 consecutive symbols. It had also been assumed that a more solid channel training would be
achieved at the super-frame level by a repetition of two training symbols in the superframe header, each
containing 2 PN-sequences. As mentioned before, it seems that in order to meet the QoS level for realtime applications such as VoIP, such more robust training sequence at each super-frame will not be
sufficient.
An interesting situation arose in London where 802.11 tenants claimed that robust synchronization and
channel training schemes based on preamble symbols would be needed while the 802.16 tenants were
claiming that was not necessary and that less robust schemes such as the one used in 802.16 based on
spread pioot carriers should be sufficient. Should 802.22 adopt a very robust synchronization and
channel training schemes based on preambles used for “fixed” operation in 802.11 or less robust
schemes that seem to be sufficient for “mobile” operation in 802.16? It would be useful here to have an
assessment of the relative robustness on 802.11 versus 802.16 systems in a same 802.22 channel
environment to see their relative performance. See comments #494, p.59, #144, p.60 and #495, p. 59.
Way forward: A decision has to be taken as to the need for the robust 2-training symbol sequence à la
802.11 in the super-frame header. This 2-training symbol sequence had some support at the super-frame
level because the overhead would have been small but if it is to appear at the beginning of each frame,
the overhead would no longer be negligible. The ETRI simulation results showed that the 240 pilot
carriers per symbol were sufficient and they indicated that in order to improve the transmission
Submission
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Gerald Chouinard, CRC
March 2007
doc.: IEEE 802.22-07/0063r2
granularity, only 3 symbols out of the 7 symbols that would be needed to completely recover all the pilot
carriers could be acquired with a loss of 0.7 dB in performance. Such loss would not be as great in the
downstream direction since the preamble symbol can be used to improve the channel estimation and that
interpolation in the frequency domain can be done. The use of pilot carriers spread over 7 symbols
rather than training symbols at the beginning of the bursts will result in a latency of 7 * 336= 2.35 ms
accompanied with the corresponding buffering at the reception. A final decision will be needed between
the training symbols alternative and the spread pilot carriers. There is no clear reason why the two
options should be maintained in the initial draft standard if they both give equivalent channel training
information.
4. Upstream symbol/carrier tiling
In the case where the previous item is resolved in the direction of the use of pilot carriers spread over a
number of symbols for channel estimation, the exact number of symbols and adjacent carriers over
which the upstream transmission takes place and, hence over which the local channel response can be
interpolated, that is the tiling pattern, needs to be decided upon. The CPE will be programmed to use a
succession of these tiles along with the pilot carriers located within these tiles to transmit its sub-channel
information and provide the base station with the channel state information needed for proper
demodulation.
Observations: The tile size that was assumed in the ETRI simulations and that is used in the OFDMA
Parameters spreadsheet is 7 symbols by 1 carrier. This tile contains one pilot for a ratio of pilot to data
capacity of 1/6. Eli Sofer has proposed the consideration of a 3 symbols by 3 carriers tile corresponding
to the 802.16e upstream OPUSC option which gives a 1/8 pilot to data capacity. These two tile sizes
need to be evaluated with the four channel models contained in document 22-05-0055-070000_WRAN_channel _Modeling.doc before conclusions can be reached on their relative merit.
Simulation results are expected from ETRI and Runcom. See comment #13, p. 39, #149, p. 60.
Way forward: Once the results of the simulations are known, the group should be in a position to take a
decision based on the best fit of the tile size with the transmission channel time and frequency spreading
characteristics expected in the TV bands. A proper trade-off between this best fit, the pilot carrier
overhead and the transmission granularity should be found to arrive at the final decision.
[8 March 07] John Benko will propose a 1X5 tile for upstream.
5. Subchannelization: carrier allocation pattern
In the initial joint proposal, two main carrier allocation patterns were proposed:
-
distributed carrier permutation
-
adjacent carrier permutation (frequency selective diversity)
The first one randomizes the carriers assigned to the different CPEs over the 6 MHz channel whereas the
second one can take advantage of the channel state information to allocate the carriers in the portion of
the 6 MHz channel that has the best performance for each CPE, therefore allowing a potential
improvement of the overall transmission performance.
Observations: There is however a drawback to the adjacent carrier permutation in the upstream
direction because of the possible concentration of CPE power over a 200 kHz bandwidth which
corresponds to the bandwidth that will affect to interference into Part 74 wireless microphones. The
Submission
page 5
Gerald Chouinard, CRC
March 2007
doc.: IEEE 802.22-07/0063r2
difference between the carriers transmitted by a CPE being concentrated in 200 kHz rather than being
spread over the entire TV channel corresponds to a worsening of the potential interference of some 10
dB at the edge of the WRAN coverage when the CPE tries to transmit its rated maximum capacity of
384 kbit/s using a 4 W EIRP. See comments 495, p.59, 145, 149, 151, 150, 147, p.60, 152, 146, p.61.
Way forward: This 10 dB worsening of the potential interference towards the Part 74 wireless
microphone incumbents needs to be considered carefully. This would tend to favor a simple
randomization of the carriers transmitted from a CPE across the TV channel. The question to be
addressed is whether the first version of the 802.22 standard should be limited to the distributed carrier
permutation on both upstream and downstream for simplicity sake.
It would be useful at the time of the decision to have indications of the extent of improvement that the
adjacent carrier permutation could provide on the downstream and on the upstream as well as the extra
MAC messages that would be required for proper signaling and operation such as the transmission of the
channel state information from the CPE to the base station, and the complexity of the scheduling
optimization needed at the base station to share the channel capacity among all active CPEs while taking
advantage of the potentially better performance of all these channels at the same time.
6. Data to Carrier time/frequency interleaving schemes
During the London session, Patrick Pirat raised the point about clarifying the detailed mapping of the
data bits in terms of randomization over symbols and carriers. If such randomization is done in two
steps, the randomization achieved through step one may be defeated by the second step of of the
process. The standard therefore needs a fully understood and described data mapping in terms of time
and frequency interleaving schemes. Draft 1.0 should contain the details of the sub-channel block sizes
per frame and a detailed description of the interleaving schemes.
Observations: It seems that this aspect has not been fully studied even though some assumptions must
have been made to be able to simulate the performance of the various sets of PHY parameters in actual
multipath channel conditions. It was noted that proper bit interleaving to map to the frequency
time multiplex is needed for cyclic FEC codes while LDPC would already include such interleaving.
The effect of truncating sub-channel capacity in order to increase the system granularity as suggested by
ETRI (3 symbols instead of 7) needs to be studied as to its impact on the data randomization. See
comment #144, #145, #147, #150 and #151, Page 60.
Way forward: Work on this item should take place between now and the March session in Orlando so
that a proper description of the interleaving schemes (data bit mapping to frequency/time physical
multiplex) can be inserted in the Draft 1.0 in time for the WG balloting. Someone (Patrick Pirat?)
should take the lead on this and form a small specialized ad-hoc group to discuss this matter and come
up with a detailed section to be added to the Draft 1.0 in March.
7. Forward Error Correction
The current Drat 0.2 includes the convolutional coding and Viterbi decoding scheme with puncturing for
the inner FEC. [8 March 07] . No outer code is needed..
A motion was passed in London to include the consideration of advanced inner coding FEC schemes in
the preparation of Draft 1.0. The joint proponent team had identified three other FEC schemes:
- LDPC
- Shortened block turbo code
Submission
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Gerald Chouinard, CRC
March 2007
-
doc.: IEEE 802.22-07/0063r2
Duo-binary convolutional turbo code (CRSC)
Observations: An ad-hoc group was formed in January 2006 during the Hawaii session to evaluate the
relative merits of each of these FEC schemes but the pace has been slow due to the expectation that only
the first scheme would be considered in the preparation of the first draft standard. Now that this has
been put as a feature to be resolved in the first draft, simulation results are required urgently.
Way forward: The work in the ad-hoc group needs to be accelerated a.s.a.p. and simulation results
using the latest set of PHY parameters and the 4 channel models need to be produced before the March
session so that a decision can be made in time for the Draft 1.0. To keep the standard simple, all efforts
should be made to arrive at one FEC mode rather than multiple options.
From comment resolution process (22-07-0063-00-0000_WGR2_Cmt_DB.pdf, page 45), once the
selection of the FEC code is made, Table 29 of the draft standard will need to be updated.
I2R does not agree with Carl's interpretation of the London motion which would only allow
consideration of LDPC and turbo code and not shortened sequence as potions. If we go with the motion,
the mandatory convolutional coding and the three optional codes would need to be included in the
standard. [8 Feb.07]
[8 Feb.07]
[8 March 07] Possible presentation by John Benko if time permit. Bit interleaving may be affected by
the type of FEC scheme.
8. Super-frame
Need to address comments on the need of super-frames (#247, p.57). Need to identify the payload of
the super-frame (one symbol at the beginning of the first frame in the series of 16 with a special PNsequence and one symbol less in this first frame to keep the frame length at 10 ms. Is a special superframe preamble needed at the receiver for frequency and time synchronization (initial burst detection,
AGC tuning, coarse frequency offset estimation and timing synchronization)? What is the spreading
method for the SCH? (Comments: #187, p.10, #93, p. 28, #525, #231, p.37, #179, #16, #17, p.41, #247,
p 57, #493, #491, #96, p.58, #115, #250, p.61)
9. TDD versus TDD/FDD
Should the first draft standard include what is necessary for a FDD operation? ETRI is to bring more
supportive information on this topic. (Comment: 39, p.20, 252, p.62)
[8 March 07] ETRI will distribute a document on FDD.
9. CBP PHY design
Need for very robust synchronization and preamble generated at the transmitting CPE aimed at
synchronizing another CPE that will receive this CBP upstream burst. Need to signal to some CPEs the
need to monitor the channel with their WRAN receive chain during the upstream sub-frame and report
on the received information. Need to specify a specific set of carriers to be used by the CBP to carry the
CPE-to-CPE geolocation information. Comment: 191, p.14, 504, p.21, 217, 213, 215, p.23, 219, 221,
207, p.24, 220, 208, 210, 211, p.25, 212, 42, 100, p.26, 216, p.27, 46, p.33, 53, p.35, 87, 11, p.39, 14, 15,
p.40, 180, p.43, 184, p.49)
10. Ranging methods
Submission
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Gerald Chouinard, CRC
March 2007
doc.: IEEE 802.22-07/0063r2
See comment 336, p.59.
11. Power control method
See comments #137, p.8, #343, #509, p.9, #337, p.59.
12. OFDMA subchannellization
See comment: 12, 13, p.39.
13. WRAN system performance at edge of coverage
The system is designed to provide the required capacity as stated in the RFD, that is 384 kbit/s at the
edge of coverage with 4 W maximum EIRP. In the WRAN reference model spreadsheet (22-04-000215-0000_WRAN_Reference_Model.xls), this is obtained by setting the required return bit rate at 384
kbit/s and finding the required minimum field strength at the base station receiver for a given set of
receiver assumptions and a Eb/No of 5.6 dB which corresponds to QPSK, rate= 1/2 performance in a
Gaussian channel. The distance is then found using the ITU-R P.1546 propagation model for 4 W
transmit power from the CPE for a service availability of F(50, 99.9). This results in a reach of 31 km
for a 75 m base station antenna height and a 10 m CPE antenna height.
For the set of current OFDMA parameters (22-06-0264-02-0000_OFDMA_Parameters.xls), if the entire
frame payload is assigned to the upstream capacity (i.e., 27 symbols for CP= 1/8), 389 kbit/s can be
transmitted using 6 sub-channels, that is 168 carriers, 1/10th of the total number of carriers in the
OFDMA multiplex. Lower capacity can still be achieved from further away CPEs until it hits the
capacity of a single sub-channel (i.e., since transmission cannot be reduced to a fraction of a subchannel), that is an additional propagation marging of 7.8 dB for a capacity reduced to 64.8 kbit/s.
With respect to the downstream, if the base station EIRP is limited to 4 W, the shared downstream
payload at the limit of coverage will be 389 kbit/s transmitted on 168 QPSK modulated carriers
occupying the total 27 symbols in a frame if the data has to be sent to CPEs located at the edge of the
coverage area because of the reciprocal RF path, and this shared payload could decrease with distance
beyond the edge of coverage similar to what is described above for the CPE. If the base station EIRP is
allowed to increase beyond 4 W, then the entire set of OFDM carriers (i.e., 10 times more) could be
transmitted to serve more CPEs at the edge of coverage. A 40 W base station EIRP would allow the
transmission of 3.888 Mbit/s toward CPEs at the edge of the coverage area if the frame is fully allocated
to downstream traffic.
In response to comment: #483, p.56, #494, p. 59.14. Other comments from London session
[1] Proposals accepted / Need follow-up
Comment #484, Page 57; Comment #492, Page 58
Comment #152, Page 61
Comment #148, Page 61
[2] Accept in Principle / Defer under discussion
Comment #483, Page 56 - discussion on more robust coding techniques and include remark about EIRP
and BS height.
Comment #336, Page 59 - discussion on ranging.
___________________________
Submission
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Gerald Chouinard, CRC